feline hyperthyroidism

Question 1

feline Hyperthyroidism
- how common?
- age?
- usual cause?

Answer 1

Most common endocrine disease of middle aged-older cats
◦ Mean age at presentation is 12-13 y
◦ <5% are <8 years at diagnosis
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Disease in one/both lobes of thyroid gland
◦ Usually adenomatous hyperplasia
◦ Overproduction of thyroid hormone
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Pathogenesis uncertain
◦ Risk factors – genetic, environmental, nutritional, other?

Question 2

hyperthyroid feline clinical presentation - common and less common clinical signs

Answer 2

COMMON:
* Weight loss
* Polyphagia
* Unkempt haircoat
* Polyuria/polydipsia
* Increased activity, “nervous”
* Intermittent vomiting/diarrhea
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LESS COMMON:
* Lethargy
* Decreased appetite

Question 3

common and less common PE abnormalities for hyperthyroid feline:

Answer 3

COMMON:
* Palpable thyroid
* Decreased BCS
* Hyperactive, impaired tolerance to stress
* Tachycardia
* Hair coat changes (dry, dander, alopecia)
* Heart murmur
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LESS COMMON:
* Arrhythmia, heart murmur
* Hypertension
* Lethargy, weakness
* Small kidneys

Question 4

“Apathetic” Hyperthyroidism - how common? what is this?

Answer 4

10% hyperthyroid cats “apathetic”
* Depressed
* Anorexia
* Weight loss
* May have concurrent illness (e.g., renal, heart disease)

Question 5

hyperthyroid feline Laboratory Results

Answer 5

CBC
* Usually normal
* Possibly increase in PCV / RBC numbers and increase in MCV
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Biochem
* Mild to moderate elevation in liver enzymes (90% of cats)
* Signs of concurrent kidney disease possible
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Urinalysis
* Usually normal, unless other diseases present

Question 6

Diagnosing Hyperthyroidism - tests

Answer 6

Total T4 extremely specific
* 10% have normal T4 (typically upper end of normal)
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Other diagnostics:
* Investigate for concurrent disease
* Investigate for consequences of hyperthyroidism

Question 7

feline hyperthyroid concurrent conditions

Answer 7

◦ Heart disease
◦ Hypertension
◦ Chronic Kidney Disease

Question 9

Thyrotoxic Cardiomyopathy - what is this? clinical signs?

Answer 9

• Hypertrophic cardiomyopathy secondary to excess thyroid hormone levels
• Dilated cardiomyopthy less common
• Congestive heart failure possible, but seems uncommon
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  Clinical signs:
• Tachycardia
• Less commonly: murmur, arrhythmia, signs of CHF

Question 10

Systemic Hypertension relation to hyperthyroidism? clinical signs? possible sequelae?
can we reverse this?

Answer 10

• Due to excess thyroid hormone or concurrent kidney disease
• Usually clinically silent
• Could cause retinal hemorrhage, detachment (uncommon)
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  Reversible, if only due to hyperthyroidism
• Could require some temporary treatment of hypertension
• E.g.,BP > 180mmHg, or concerns of retinal changes)

Question 10

Chronic Kidney Disease relationship to hyperthyroidism

Answer 10

• Hyperthyroidism and CKD both common conditions of older cats
• Can be present in the same cat
• Hyperthyroidism does not cause CKD
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  Hyperthyroidism will increase blood flow to the kidneys
  ◦ Increased blood flow increases renal filtration ability
  ◦ Therefore could mask azotemia/signs of CKD
• Once euthyroidism achieved
  > True reflection of renal status

Question 11

Therapeutic Options for hyperthyroidism

Answer 11

◦ Surgical
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◦ Medical
> Methimazole
> Radioactive Iodine
> Nutritional management – low iodine

Question 12

Thyroidectomy - possible complications?

Answer 12

◦ Minimal complications
◦ Ectopic hyperplastic tissue – recurrence

Question 13

Methimazole - what does it do?

Answer 13

Concentrated by thyroid gland
* Inhibit T3 & T4
* Potentially reversible

Question 14

Methimazole Side Effects - when are they seen? what are they? how common?

Answer 14

Usually occur in first 4-8 weeks of treatment
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Gastrointestinal signs (~10% of treated cats)
* Vomiting, diarrhea
* Self-limiting (temporary dose decrease)
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Blood cell abnormalities – most commonly neutropenia (<10% treated cats)
* Usually resolves when medication discontinued
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Facial excoriation (<5% of treated cats)
* Scabbed lesions on head, pinnae & neck
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Hepatotoxicity (2% of treated cats)
* Increases in liver enzyme values, signs of illness
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Significant side effects will require change to another form of treatment

Question 15

Radioactive Iodine (131I) - safety and effectiveness? admin strategy?

Answer 15

Safe, simple, effective
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Usually administered SQ (or IV)
◦ Cats isolated (7-10d)
◦ Variable protocol per facility
◦ Some regulatory changes have limited availability of this treatment at present

Question 16

Nutritional Management for hypothyroidism
- considerations for diet
- speed of efficacy

Answer 16

Iodine restricted prescription diet
* Sole nutrition source
* Caution if other medications/supplements
* Compliance is critical
* Cannot go outdoors
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T4 slower to return to normal
* 8 weeks (versus 2-4 weeks with methimazole)

Question 17

goal of methimazole therapy

Answer 17

◦ Goal is mid-range TT4 (15-40 nmol/L)
◦ Recheck in 2-4 weeks

Question 18

CKD & Hyperthyroidism after thyroid issues are treated
- what is revealed

Answer 18

◦ Once thyroid levels are controlled, GFR returns to normal
◦ Unmasks the true kidney function
◦ ~20% of patients will show azotemia after hyperthyroidism is
controlled
◦ CKD was present all along, but masked by the elevated thyroid hormone level +/- cachexia

Question 19

prognosis for hyperthyroid cats with azotemia before treatment vs those that develop azotemia after treatment

Answer 19

◦ Hyperthyroid cats with azotemia before treatment have poorer prognosis
> MST 178 days
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◦ Hyperthyroid cats that develop azotemia only after treatment
> Prognosis similar to those that don’t develop azotemia
> MST 905 days

Question 20

Managing CKD & Hyperthyroidism - goals for TT4 levels

Answer 20

◦ Keep TT4 in middle of reference range
> Low-normal or low T4 will drop GFR, associated with poorer survival
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No advantage to keeping the TT4 at high levels
◦ Urea and creatinine will improve but true kidney status does not improve
◦ Prolonged hyperthyroidism likely damages kidney

Question 21

Long-term Monitoring for hyperthyroid cat?
Hyperthyroid cats developing azotemia after treatment - are they candidates for permanent treatment options?

Answer 21

◦ Once TT4 is stable, decrease frequency of rechecks
◦ Monitor kidney status as per CKD guidelines, if applicable
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◦ Hyperthyroid cats developing azotemia after treatment still good candidates for permanent treatment options (e.g., I-131)

Question 22

Prognosis: Hyperthyroidism

Answer 22

Survival times 2-5+ years
◦ Hyperthyroidism usually does not cause death
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Azotemia prior to treatment
◦ Decreases prognosis

Question 23

Name a common drug that could cause low T4 levels in a dog without hypothyroidism

Answer 23

◦ Steroids
◦ Phenobarbital
◦ Sulfonamides

Question 24

Name a common abnormality on the biochemical profile in dogs with hypothyroidism?

Answer 24

• Decreased total T4
• Increased cholesterol (~75% dogs)
• Increased triglycerides
• Increased liver enzymes (~30% dogs)

Question 25

What is a common biochemical profile abnormality with hyperthyroidism, usually resolves with treatment of thyroid disease?
◦ Elevated ALT
◦ Hypertriglyceridemia
◦ Hypoglycemia

Answer 25

◦ Elevated ALT
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* Mild to moderate elevation in liver enzymes (90% of cats)
* Signs of concurrent kidney disease possible

Question 26

Which of the following is a common side effect of methimazole therapy?
◦ Kidney damage
◦ Lameness
◦ Vomiting

Answer 26

Gastrointestinal signs (~10% of treated cats)
* Vomiting, diarrhea
* Self-limiting (temporary dose decrease)